Of course you exist! You're a perceiver, right? The question is, do I exist right now? How about now? Do you know? Do you perceive me? That's the thing for Berkeley. He thinks, "Well, I'm not really sure that Paul exists right now, so it's a good thing reality isn't contingent upon MY perception!". Bear in mind, Berkeley was a Bishop, so his philosophy seems to fall in line with some sort of meticulous theology as well. He thinks that when we open our eyes, it's obvious that we can't manipulate the things that we perceive, so reality is contingent upon a higher perceiver than ourselves. God.